Thermal type air flow rate measurement apparatus (a thermal type air flow meter), which has thermal sensitive resistors such as a heating resistor and temperature compensating resistor having a temperature characteristic, is cable of directly measuring a mass air flow rate. Therefore, it is widely diffused to measure a flow rate of an intake air for an internal combustion engine of a motor vehicle or the like. A signal of the measured air flow rate is provided for, e.g., calculating a fuel injection quantity of an electronically controlled fuel injector.
As a thermal sensitive resistor such as a heating resistor, a hot wire flowmeter, which is configured by winding a platinum wire on a bobbin and coating the wound bobbin with glass, is widely diffused for practical use. In addition to that, in recent years, a thin-film flowmeter in which thin film resistors is formed on a ceramic or silicon substrate and a semiconductor flowmeter made of, e.g., polysilicon have been proposed.
Generally, there are two kind of flow rate measurement ways: one is a measurement way of controlling a heating volume of a heating resistor on a flow passage so as to maintain a predetermined temperature difference between the heating resistor and a temperature compensating resistor, and directly sensing a current flowing through the heating resistor to measure the flow rate; the other is a measurement way of arranging temperature sensing resistors upstream and downstream of a heating resistor respectively and obtaining a temperature difference between both temperature sensing resistors to measure the flow rate. Both measurement ways make use of the resistance temperature characteristic that changes when the thermal sensitive resistors exchange heat with the fluid.
When using flow rate sensing elements having a relatively large response delay of its output, it is necessary to correct the response delay by inverse conversion of a time constant corresponding to the response delay. This correction of response delay is performed on the sensor side at a stage (pre-processing) before a flow rate signal is taken in to a controller (e.g., an engine control unit described in, e.g., Japanese Laid-Open Patent Publication No. Hei 8-62012 and Japanese Laid-Open Patent Publication No. Hei 11-14418) or performed on the engine control unit side (it's described in, e.g., Japanese Laid-Open Patent Publication No. Hei 6-10752 and Japanese Laid-Open Patent Publication No. 2003-13789).
The former is an example of digitally correcting the characteristic of the flow rate meter (sensor) and the latter is an example of a manner of correcting a measurement error due to a response delay of the sensor, on the engine control unit side. Both correcting ways are used to improve the response delay of the sensor having a large response delay.
A thermal type flow rate sensor has a nonlinear output characteristic and the output may have a pulsation due to a backflow from an engine or the like. These factors cause an error in an output signal. In order to such an error, the conventional technical example, in which the output signal from the sensor is digitally corrected by the sensor circuit before it is output to the engine control unit, is disclosed in, e.g., Japanese Laid-Open Patent Publication No. Hei 11-94620.
Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 8-62012
Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 11-14418
Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 6-10752
Patent Document 4: Japanese Laid-Open Patent Publication No. 2003-13789
Patent Document 5: Japanese Laid-Open Patent Publication No. Hei 11-94620